The present invention relates generally to a ballast circuit for fluorescent lamps. More particularly, this invention relates to a self-oscillating electronic ballast circuit with cathode protection during normal operation and ballast protection during no-lamp and cathode failure conditions.
FIG. 1 shows a ballast circuit 100 with a series-resonant parallel-loaded electronic ballast having an inherent open-cathode protection function. The open-cathode protection function is provided by placing a resonant capacitor 112 between the two cathodes 114, 116 of the fluorescent lamp 118. If the fluorescent lamp 118 is removed from the ballast circuit 100, or if one or two of the cathodes 114, 116 fail (i.e., cathode current path opens), the resonant inductor 120 is disconnected from the resonant capacitor 112. With the resonant circuit disconnected, the self-oscillating electronic ballast is disabled. Upon replacing the lamp 118, the ballast circuit 100 will resume its normal operation. However, the resonant current that flows through the resonant capacitor 112 during normal operation also flows through each of the cathodes 114, 116. The continuous resonant current can cause overheating of the cathodes 114, 116, reduces the life of the cathodes 114, 116, and reduces the lumens per watt (LPW) of the lamp 118.
FIG. 2 shows another ballast circuit 200 with a series-resonant parallel-loaded electronic ballast with reduced cathode current and a corresponding reduction in power dissipation by the cathodes during normal operation. The ballast circuit 200 achieves reduced cathode current by splitting the resonant capacitance between two capacitors (i.e., capacitor 212 and capacitor 214). Capacitor 212 is between the two cathodes 216, 218 of the fluorescent lamp 220, like in FIG. 1. However, capacitor 214 is in parallel with the two cathodes 216, 218. In this arrangement, the current that flows through cathodes 216, 218 during normal operation of the lamp 220 is reduced. Likewise, the corresponding power dissipated by the cathodes 216, 218 during normal operation is reduced. However, under a no-lamp condition the resonant circuit formed by capacitor 214 and resonant inductor 222 is still intact and continues to conduct current. Furthermore, with the lamp 220 removed, the resonant circuit will have a higher voltage and higher current than with the lamp 220 installed. This could result in damage to the ballast under the no-lamp condition.
In one aspect of the present invention a ballast circuit for driving a fluorescent lamp is provided. The ballast circuit comprises: a self-oscillating circuit; and a series resonant circuit.
In another aspect of the present invention a ballast circuit for driving a fluorescent lamp is provided. The ballast circuit comprises: a self-oscillating circuit; a resonant inductor; a resonant capacitor; a first diode; and a second diode.
In another aspect of the present invention a series resonant circuit for a ballast circuit, wherein the ballast circuit is adapted for driving a fluorescent lamp is provided. The series resonant circuit comprises: a resonant inductor; a resonant capacitor; a first diode; and a second diode.